If you are thinking full-frame, the following lenses are very interesting:

-20mm f/2.8 (still available new): good results, better corners than the CZ16-35 and much, much lighter

-24-105 f/3.5-f/4.5: this lens is very useful because the range is perfect and it is incredibly small. It is a good walk-around lens on the A900. People criticize its sharpness, but used at f/8 it is quite good and full open the center resolution is great (which is fine in practice).

-50 f/1.8: just as good as the f/1.4 in practice and a lot cheaper

-85 f/1.4: I actually prefer this lens to the CZ version, full open it has a more dreamy look which is perfect for portraits

-135 f/2.8: small and light, and you would be hard pressed to see the difference with the CZ at f/4

-200mm f/2.8 and x1.4 converter: a perfect 200mm and a very good 300mm in a very compact package